The trajectory of the map you posted, points toward an area of the country where there are numerous small shallow deposits of gravel. I’ve often wondered how these deposits were formed, when rock to do so is hundreds and hundreds of miles away. I wonder if maybe the impact could have thrown the gravel there from the Great Lakes region? The gravel consists of sedimentary kind of rock with fossilized sea shells and etc., mixed with a little quartz and flint. No limestone or granite, IIRC, to speak of.